The present invention is particularly applicable to inductive output tetrode devices ( hereinafter referred to as "IOT's"). An IOT device includes an electron gun arranged to produce a linear electron beam and a resonant input cavity at which a high frequency r.f. signal to be amplified is applied to produce modulation of the beam at a grid of the electron gun. The resultant interaction between the r.f. energy and the electron beam produces amplification of the high frequency signal which is then extracted from an output resonant cavity.
One known IOT device is schematically illustrated in longitudinal section in FIG. 1. The IOT includes an electron gun 1 which comprises a cathode 2, an anode 3 and a grid 4 located between them. The electron gun is arranged to produce an electron beam directed along the longitudinal axis X--X of the arrangement. The IOT also includes drift tubes 5 and 6 via which the electron beam passes before being collected by a collector (not shown). A cylindrical annular input cavity 7 is arranged coaxially about the electron gun 1 and includes an input coupling 8 at which an r.f. signal to be amplified is applied. An output cavity 9 surrounds the gap between the drift tubes 5 and 6 and includes a coupling loop 10 via which an amplified r.f. signal is extracted and coupled into a secondary output cavity 11 from which the output signal is taken via an output coupling 12.
The input cavity 7 comprises an inner body portion which includes two transversely arranged annular plates 13 and 14. The first plate 13 is connected via conductive spring fingers (not shown) to a tubular member 15 which mechanically supports the cathode 2 and is maintained at cathode potential. The other transverse plate 14 is connected via spring fingers to a support 16 of the grid 4 and is at the grid potential. The input cavity 7 also includes an outer body portion which is electrically separate from the inner body portion and comprises transverse annular plates 17 and 18 connected by a cylindrical axially extensive wall 19 and arranged coextensively with part of the plate 13. The outer body portion also includes further transverse plates 20 and 21 connected by a cylindrical wall 22 which are partially coextensive with the plate 14 which is electrically connected to the grid 4. These two interleaved structures acts as r.f. chokes to reduce leakage of the applied high frequency energy into the region between the grid 4 and anode 3 and to the outside of the cavity 7. The cavity 7 further includes an axially extensive portion 23 having a movable tuning device 24 to permit the frequency of operation to be altered. It also includes a cylindrical wall 25 connected to the plate 21 and being axially extensive in the region between the supports 16 and 26 of the grid 4 and anode 3, respectively.
Dielectric material 27 is located between the interleaved transverse plates of the inner and outer body portions to provide structural support and electrical insulation.
Ceramic cylinders 28 and 29 surround the electron gun assembly and define part of the vacuum envelope.
In use, a d.c. voltage, typically of the order of 30-40 kV is established between the cathode 2 and the anode 3 and an r.f. input signal is applied between the cathode 2 and the grid 4. The r.f. choke defined by plates 14, 20 and 21 reduces coupling between the cathode/grid region and the anode 3. However, in some circumstances this may be insufficient to completely prevent leakage of r.f. energy and coupling between the two regions and, as a result, unwanted oscillation of the electron beam may occur. Such oscillation may not only decrease the operating efficiency of the tube but may also cause arcing within the tube sufficient to damage or disable it.
The present invention seeks to provide an improved electron beam tube in which the problem of unwanted oscillation is reduced or eliminated hence permitting devices to operate at higher maximum operating frequencies. The invention is particularly applicable to IOTs but may also be advantageously employed in other types of electron beam tubes.